Digital still cameras and digital video cameras have been rapidly widespread, thanks to improvement in the degree of integration and low cost of image pick-up sensors and signal processing circuits, such as Charge Coupled Devices (CCDs) and Complementary Metal-Oxide Semiconductors (CMOS).
Also, in recent years, cellular phones, Personal Digital Assistants (PDAs), and other terminals having incorporated therein a digital camera have become quite popular. In the future, it is predicted that digital cameras will be further widespread also in the fields of monitor cameras and vehicle-mounted cameras.
Such a digital camera includes an image pickup device. In general, the image pickup device includes an optical system, a barrel, and an image pickup sensor.
Most digital cameras in recent years perform zooming and focusing in a motor-driven manner. Such motor-driven zooming and focusing are performed by using a motor to drive a mechanism of holding a predetermined lens element included in an optical system so as to move the lens element to a direction parallel to an optical axis.
However, when the holding mechanism is driven by a general-purpose brush less motor, a space has to be provided to the lens barrel for placing the motor, thereby increasing the size of the lens barrel. Moreover, a transmitting mechanism, such as a gear, for transmitting the rotation of the motor is required, thereby increasing the size of the lens barrel and generating noise.
In order to get around the problems, a technology is suggested in which the holding mechanism is driven by using an electromagnetic motor that includes a cylindrical stator with an optical axis of a lens element as a center axis and a cylindrical rotor that is coaxial with the stator (Japanese Patent Laid-Open Publication No. 2000-131586 and Japanese Patent No. 3232517).
With the use of such an electromagnetic motor including a cylindrical rotor, a space for placing a motor is not required. Also, the structure of the transmitting mechanism including a gear for transmitting the rotation of the motor can be simplified. Therefore, the lens barrel can be downsized in a direction perpendicular to the optical axis. Also, noise generated at the time of driving can be reduced.
In general, however, in such an electromagnetic motor including a cylindrical rotor, the rotation of the rotor has to be supported at a position away from the center of rotation. Therefore, the rotor is not stable in position with respect to the stator, and the positioning accuracy of the rotor is low.
In order to get around the problems, in the electromagnetic motor disclosed in Japanese Patent Laid-Open Publication No. 2000-131586, the length of a bearing portion for supporting the rotation of the rotor is increased in a direction parallel to the optical axis, thereby stabilizing the rotor in position. Therefore, in the lens barrel disclosed in Japanese Patent Laid-Open Publication No. 2000-131586, it is difficult to make the lens barrel compact in size in the direction parallel to the optical axis.
On the other hand, in the electromagnetic motor disclosed in Japanese Patent No. 3232517, a bearing for supporting the rotation of the rotor is provided between the rotor and the stator, thereby stabilizing the rotor in position. Therefore, in the lens barrel disclosed in Japanese Patent No. 3232517, it is difficult to make the lens barrel compact in size in the direction perpendicular to the optical axis.
As such, in the conventional lens barrel provided with an electromagnetic motor including a cylindrical rotor, it is difficult to make the lens barrel compact in size in the direction parallel or perpendicular to the optical axis.